By the term "osteosynthesis", one understands fixation of bone parts by means of inserted implants taking place after a bone fracture. In this connection, reference is made to, for example, the book "Manual der Osteosynthesis" by M. E. Muller, M. Allgower and H. Willenegger, which was published by Springer-Verlag in 1969. Various types of osteosynthesis plates are described there and it is also explained how these have to be screwed fast to the parts of a broken bone. The bearing surfaces, by which the plates rest on the bone, can be flat or slightly curved. The screw holes are formed either by drilled holes or by slots. The so-called dynamic compression plates are of great importance. In these, at least one of the screw holes is formed by a slot, which extends in the longitudinal direction of the plate and which is inclined at the one end. When a screw is inserted at the inclined end of the slot, it causes a displacement of the plate in its longitudinal direction. It can thereby be attained that the bone parts at the location of the fracture are firmly pressed against each other. Such compression plates and their uses are, apart from the already mentioned book, described in the book "Die dynamische Kompressionsplatte DCP" by M. Allgower, L. Kinzl, P. Matter, S. M. Perren and T. Ruedi, published by Springer-Verlag in 1973, as well as in the publication "A dynamic compression plate" by S. M. Perren, M. Russenberger, S. Steinemann, M.E. Muller and M. Allgower; Acta Orthop, Scand. Suppl. 125, 29, 1969. The constructions in Swiss Pat. No. 462,375 and the corresponding U.S. Pat. No. 3,552,389 are the basis of the dynamic compression plates described particularly in these documents. With known plates displaying only round holes, the compression and re-positioning of the fracture zone is attained by means of a special tension device which is anchored to the bone and then engages the bone plate screwed in a preceding step to the other bone fragment.
These previously known plates of metal usually have the same crosssectional dimensions along their entire length and are thus formed by a profile rod. These plates have, in general, well stood the test. When the broken bone, the parts of which are held together by the plate, because of any kind of peculiarity does not unite together or when the plate is not correctly fastened during the operation, it can happen in exceptional cases that the plates break. These cases account for some 0.5 to 2% of osteosynthesis fracture treatments. The breakage point of the plates in this case always lies at one of the screwholes, since the cross-section of the plate, i.e. the strength, is reduced at these locations.
From U.S. Pat. No. 3,463,148, there is already known a plate in which the height, i.e. the cross-sectional dimension parallel to the passage direction of the screw holes, is greater in the region of the screw holes than in the regions between them. The variation of the height of the plate is so arranged that the material of the plate displays a substantially constant cross-sectional area over the entire length of the plate. This constant cross-sectional area should result in a substantially constant strength over its entire length.
The cross-sectional area determines the strength of the plate in tensile stress. When the height of the plate is now so increased at the screw holes that the cross-sectional area of the material remains constant, the plate indeed receives a constant tensile strength over the entire length. However, an unnecessary increase in the bending stiffness results at the screw holes. As will still be explained later, the tensile strength is on the one hand however only of secondary importance to fatigue fracture strength, while the bending stiffness on the other hand may not be varied arbitrarily.
For the rest, the previously known concept results in various disadvantages depending upon the remaining demands on the plate. The crosssectional dimensions of the plate must naturally be adapted to the dimensions of the bones. For fractures of the tibia (the shin bone), compression plates are now for example used, the width of which amounts to about 11 millimeters. The width of a slot of such a compression plate then varies between 5.5 and 8 millimeters. For such a compression plate, the width of the slots is thus relatively great in relation to the width of the plate. When, in the case of such a compression plate, it is desired to make the cross-sectional areas constant over the entire length of the plate, the height of the plate at the slots must be about 50 to 60% greater than in the remaining regions. If such a pressure plate were inserted in a bone, a considerable danger would exist that the tissue, and in particular the skin, disposed on the side of the plate remote from the bone would be damaged. A further disadvantage of this plate is that its bending stiffness to bends along a plane running parallel to the longitudinal direction of the plate and to the passage direction of the slots is considerably increased. The bending stiffness of a plate should however lie within a predetermined range for each bone. When the bending stiffness is namely too small, a bone-resorption, i.e. a dissolution of the bone, can take place in the fracture gap because of too vigorous movements of the bone parts. When the bending resistance is, on the contrary, too great, the bone is insufficiently loaded, which has the consequence of an osteoporosity, i.e. a decalcifying and weakening of the entire bone.
Previously also known in this field is a plate, namely the plate according to Sherman, the width of which measured perpendicularly to the passage direction of the screw holes is greater in the region of the screw holes formed by countersunk holes then in the remaining regions. This known plate is about 7 millimeters wide between the screw holes and about 10 millimeteres wide at the screw holes. The screw holes display a cylindrical section of 4 millimeters in diameter and a conical countersinking, the maximum diameter of which amounts to about 6.5 millimeters. A considerable danger of breakage in the region of the screw holes likewise exists in the case of this plate. Moreover, this plate is over 40% wider at the screw holes than between these. Such relatively large projections however result in a great danger of injury to the soft body tissues and the skin in the neighborhood of the plate with the movements of the patient.
According to the operational technique at present taught and applied in surgery, the plates should be inserted in such a manner that they are loaded exclusively in tension. When this is the case, fractures hardly occur in the plates of constant cross-sectional dimensions over their entire length as described in the two books already mentioned, the Swiss Pat. No. 462,375 and the U.S. Pat. No. 3,552,389. As already mentioned, the plates can however break in extreme cases. Such cases possibly arise with complicated fractures, porous bones, delayed healing or when the patient displays an extremely great body-weight or is excessively active motionally. It has now been recognised that breakage of the plate were caused not by single peak loadings, but were a consequence of the material fatigue because of repeated, relatively great loadings. It has been further ascertained through the evaluation of clinical experience that the plate breakages were caused not by tensional stresses, but by bending stresses or eventually torsion. Decisive in that case are those bending stresses which occur when the plate is bent along a plane which extends parallel to the longitudinal direction of the plate and at least approximately parallel to the opening of the screw holes, i.e. the screw axes and, indeed, above all, in the case of those bendings in which the plates at its surface remote from the bone, is stressed in tension.